1. Field of the Invention
The present invention relates to medical devices for monitoring cardiovascular properties, e.g. cardiac output (CO), stroke volume (SV), and continuous non-invasive blood pressure (cNIBP).
2. Description of the Related Art
CO is typically measured in a hospital setting and, informally, indicates how efficiently a patient's heart pumps blood through their arterial tree. More specifically, CO, with units of liters/minute, describes the time-dependent volume of blood ejected from the left ventricle into the aorta; it indicates how well the patient's heart delivers blood-borne oxygen, nutrients, and other substances to the cells in the body. CO is the product of heart rate (HR) and SV, where SV is defined as the mathematical difference between left ventricular end diastolic volume (EDV) and end systolic volume (ESV), i.e.:CO=SV×HR  (1)
Combining CO and mean arterial blood pressure (MAP) into a single value, called ‘cardiac power’ (CP), provides a particularly valuable prognostic variable for monitoring patients suffering from cardiac conditions such as congestive heart failure (CHF), and is an independent predictor of mortality that can be measured non-invasively using cardiopulmonary exercise testing. Specifically, CP is defined as:CP=CO×MAP  (2)
Measuring CO and SV in a continuous, non-invasive manner with high clinical accuracy has often been considered a ‘holy grail’ of medical-device monitoring. Most existing techniques in this field require in-dwelling catheters, which in turn can harm the patient, are inherently inaccurate in the critically, and require a specially trained operator. For example, current ‘gold standards’ for this measurement are thermodilution cardiac output (TDCO) and the Fick Oxygen Principal (Fick). However both TDCO and Fick are highly invasive techniques that can cause infection and other complications, even in carefully controlled hospital environments. In TDCO, a pulmonary artery catheter (PAC), also known as a Swan-Ganz catheter, is typically inserted into the right portion of the patient's heart. Procedurally a bolus (typically 10 ml) of glucose or saline that is cooled to a known temperature is injected through the PAC. A temperature-measuring device within the PAC, located a known distance away (typically 6-10 cm) from where fluid is injected, measures the progressively increasing temperature of the diluted blood. CO is then estimated from a measured time-temperature curve, called the ‘thermodilution curve’. The larger the area under this curve, the lower the cardiac output. Likewise, a smaller the area under the curve implies a shorter transit time for the cold bolus to dissipate, hence a higher CO.
Fick involves calculating oxygen consumed and disseminated throughout the patient's blood over a given time period. An algorithm associated with the technique incorporates consumption of oxygen as measured with a spirometer with the difference in oxygen content of centralized blood measured from a PAC and oxygen content of peripheral arterial blood measured from an in-dwelling cannula.
Both TD and Fick typically measure CO with accuracies between about 0.5-1.0 l/min, or about +/−20% in the critically ill.
Several non-invasive techniques for measuring SV/CO/CP have been developed with the hope of curing the deficiencies of Fick and TD. For example, Doppler-based ultrasonic echo (Doppler/ultrasound) measures blood velocity using the well-known Doppler shift, and has shown reasonable accuracy compared to more invasive methods. But both two and three-dimensional versions of this technique require a specially trained human operator, and are thus, with the exception of the esophageal Doppler technique, impractical for continuous measurements. CO/SV can also be measured with techniques that rely on electrodes placed on the patient's torso that inject and then collect a low-amperage, high-frequency modulated electrical current. These techniques, based on electrical bioimpedance and called ‘impedance cardiography’ (ICG), ‘electrical cardiometry velocimetry’ (ECV), and ‘bioreactance’ (BR), measure a time-dependent electrical waveform that is modulated by the flow of blood through the patient's thorax. Blood is a good electrical conductor, and when pumped by the heart can further modulate the current injected by these techniques in a manner sensitive to the patient's CO. During a measurement, ICG, ECV, and BR each extract properties called left ventricular ejection time (LVET) and pre-injection period (PEP) from time-dependent ICG and ECG waveforms. A processor then analyzes the waveform with an empirical mathematical equation, shown below in Eq. 2, to estimate SV. CO is then determined from the product of SV and HR, as described above in Eq. 1.
ICG, ECV, and BR all represent a continuous, non-invasive alternative for measuring CO/SV, and in theory can be conducted with an inexpensive system and no specially trained operator. But the medical community has not embraced such methods, despite the fact that clinical studies have shown them to be effective with some patient populations. In 1992, for example, an analysis by Fuller et al. analyzed data from 75 published studies describing the correlation between ICG and TD/Fick (Fuller et al., The validity of cardiac output measurement by thoracic impedance: a meta-analysis; Clinical Investigative Medicine; 15: 103-112 (1992)). The study concluded using a meta analysis wherein, in 28 of these studies, ICG displayed a correlation of between r=0.80-0.83 against TDCO, dye dilution and Fick CO. Patients classified as critically ill, e.g. those suffering from acute myocardial infarction, sepsis, and excessive lung fluids, yielded worse results. Further impeding commercial acceptance of these techniques is the tendency of ICG monitors to be relatively bulky and similar in both size and complexity to conventional vital signs monitors. This means two large and expensive pieces of monitoring equipment may need to be located bedside in order to monitor a patient's vital signs and CO/SV. For this and other reasons, impedance-based measurements of CO have not achieved widespread commercial success.
ICG-based methodologies for measuring CO/SV have evolved since Fuller's analysis. For example, it has recently been shown that the dimensionless peak rate of change of the trans-thoracic electrical impedance pulse variation, which is defined as the maximum value of the derivative of the ICG waveform (dZ/dt)max divided by the base impedance (Zo), is an acceleration analog (with units of 1/s2). When subjected to square root transformation this yields ohmic mean velocity [(dZ/dt)max/Zo)]0.5. These parameters are described in detail in U.S. Pat. Nos. 7,740,590 and 7,261,697, the contents of which are fully incorporated herein by reference. Reasonable facsimiles of SV can be obtained when this value is multiplied by LVET and a volume conductor (Vc) allometrically related by body mass to the intrathoracic blood volume. As compared to CO measured with TDCO and transesophageal echocardiography, good to high correlation and limits of agreement within +/−30% are reported.
While most ICG measurements are conducted on the thorax, there is good evidence in the literature implying that left ventricular SV can be obtained from the upper extremity, and specifically the brachium. For example, Chemla et al. showed that peak aortic blood acceleration is highly correlated with peak brachial artery blood acceleration (r=0.79) (see, e.g., Chemla et al., Blood flow acceleration in the carotid and brachial arteries of healthy volunteers: respective contributions of cardiac performance and local resistance; Fundam Clin Pharmacol; 10: 393-399 (1996)). This study also demonstrated that, while brachial blood velocity is affected by downstream vasoactivity, peak brachial blood acceleration is solely affected by the upstream β-adrenergic influences of cardiac impulse formation. This suggests that square root transformation of brachial (dZ/dt)max/Zo may yield accurate estimations of SV when multiplied by LVET and a Vc of appropriate magnitude. Stanley et al. showed that the maximum early systolic upslope of the transthoracic and brachial impedance changes (ΔZ) are identical, indicating that they are linearly correlated (see, e.g. Stanley et al., Multi-channel electrical bioimpedance: a new noninvasive method to simultaneously measure cardiac and peripheral blood flow; J Clin Monit Comput; 21: 345-51 (2007)). This implies that, despite being of different magnitudes, the peak rate of change of the trans-thoracic and trans-brachial impedance changes can both be used to calculate SV. Finally, Wang et al. demonstrated that impedance changes (ΔZ(t)) in the forearm are highly correlated with Doppler-derived SV, showing a correlation coefficient of r=0.86 (see, e.g. Wang et al., Evaluation of changes in cardiac output from the electrical impedance waveform in the forearm; Physiol Meas; 28: 989-999 (2007)).
CO/SV can also be estimated from a time-dependent arterial blood pressure waveform measured, e.g., with a tonometer or in-dwelling arterial catheter. Algorithms can be used to extract pulse pressure (PP) and other contour-related features from these waveforms, which are then processed to estimate CO/SV. Unfortunately both the heart and its associated vessels can function independently and sometimes paradoxically, so changes in parameters like PP may both reflect and mask changes in CO/SV. In other words, measurements of CO using time-dependent arterial waveforms represent a combination of cardiac and vascular function.
Pulse arrival time (PAT), defined as the transit time for a pressure pulse launched by a heartbeat in a patient's arterial system, has been shown in a number of studies to correlate to both systolic (SYS) and diastolic (DIA) blood pressures. In these studies, PAT is typically measured with a conventional vital signs monitor that includes separate modules to determine both an electrocardiogram (ECG) and a value for pulse oximetry (SpO2). During a PAT measurement, multiple electrodes typically attach to a patient's chest to determine a time-dependent component of the ECG waveform characterized by a sharp spike called the ‘QRS complex’. The QRS complex indicates an initial depolarization of ventricles within the heart and, informally, marks the beginning of the heartbeat and a pressure pulse that follows. SpO2 is typically measured with a bandage or clothespin-shaped sensor that attaches to a patient's finger, and includes optical systems operating in both red and infrared spectral regions. A photodetector measures radiation emitted from the optical systems that transmits through the patient's finger. Other body sites, e.g., the ear, forehead, and nose, can also be used in place of the finger. During a measurement, a microprocessor analyses both red and infrared radiation measured by the photodetector to determine time-dependent waveforms corresponding to the different wavelengths, each called a photoplethysmogram waveform (PPG). From these a SpO2 value is calculated Time-dependent features of the PPG waveform indicate both pulse rate and a volumetric absorbance change in an underlying artery (e.g., in the finger) caused by the propagating pressure pulse.
Typical PAT measurements determine the time separating a maximum point on the QRS complex (indicating the peak of ventricular depolarization) and a portion of the PPG waveform (indicating the arrival of the pressure pulse). PAT depends primarily on arterial compliance, the propagation distance of the pressure pulse (which is closely approximated by the patient's arm length), and blood pressure. To account for patient-specific properties, such as arterial compliance, PAT-based measurements of blood pressure are typically ‘calibrated’ using a conventional blood pressure cuff. Typically during the calibration process the blood pressure cuff is applied to the patient, used to make one or more blood pressure measurements, and then removed. Going forward, the calibration measurements are used, along with a change in PAT, to determine the patient's blood pressure and blood pressure variability. PAT typically relates inversely to blood pressure, i.e., a decrease in PAT indicates an increase in blood pressure.
A number of issued U.S. patents describe the relationship between PAT and blood pressure. For example, U.S. Pat. Nos. 5,316,008; 5,857,975; 5,865,755; and 5,649,543 each describe an apparatus that includes conventional sensors that measure ECG and PPG waveforms, which are then processed to determine PAT.